1. Field of the Invention
The present invention relates to a reception apparatus capable of reducing the consumed electric power, particularly a reception apparatus using a code division multiple access system in which communication information is multiplied by a spreading code to transmit, and the communication information is obtained at a reception side by multiplying a received signal by the same spreading code as a transmission side.
2. Description of the Related Art
A code division multiple access (CDMA) system is standardized in U.S.A. for a cellular mobile telephone as IS 95. The basic research and practical research of CDMA techniques have been carried out widely as an advantageous access system in the public terrestrial mobile telephone system.
A CDMA system is a communication system having the resistance to noise, in which a plurality of orthogonal (low cross-correlation) spreading codes are used to communicate in the same frequency band, and a multiple use of the frequency band allows an increase in system capacity.
FIG. 1 illustrates a block diagram of a conventional mobile radio terminal apparatus (hereinafter described as mobile station) and the explanation is given below. FIG. 1 is a schematic configuration diagram of a mobile station in IS95 system in a CDMA mobile radio system.
A signal received by antenna 301 (received signal) is inputted to frequency conversion section 302. Frequency conversion section 302 frequency multiplying the received signal by a frequency (sine wave) outputted from frequency synthesizer section 304 to convert into an intermediate frequency band from a radio frequency band with respect to the received signal. The frequency converted received signal is outputted to quadrature demodulation section 303.
A signal transmitted from a base station (not shown) or a received signal when no influence by fading in a transmission path and no multi-path delayed versions is expressed in the formulation (1) below, EQU (Received Signal)={Wd(t).multidot.D(t)+Wp(t)}.times.{PNi(t).multidot.cos (.omega.ct)+PNq(t).multidot.sin (.omega.ct)} (1)
where reference signals are PNi(t) and PNq(t), an amplitude of a reference signal is Wp(t), a carrier frequency of a base station is .omega.C, information symbol is D(t), and channel identifying code is Wd(t), in this case one channel is used for the simplified explanation.
Quadrature demodulation section 303 multiplies a received signal by a signal of cos (.omega.ct+.phi.) and a signal of sin (.omega.ct+.phi.) each outputted from frequency synthesizer section 304 for the low pass filtering processing, and generates I channel baseband signal I (t) and Q channel baseband signal Q(t) expressed in the formulations (2) and (3) below, EQU I(t)=1/2{Wd(t).multidot.D(t)+Wp(t)}.times.{PNi(t).multidot.cos .phi.-PNq(t).multidot.sin .phi.} (2) EQU Q(t)=1/2{Wd(t).multidot.D(t)+Wp(t)}.times.{PNi(t).multidot.sin .phi.+PNq(t).multidot.cos .phi.} (3)
where .phi. indicates a phase difference between a carrier of a mobile station and a carrier of a base station.
Analogue I channel baseband signal I(t) generated in quadrature demodulation section 303 is converted into a digital signal in A/D(Analogue/Digital) conversion section 305a. Analogue Q channel baseband signal Q(t) generated in quadrature demodulation section 303 is converted into a digital signal in A/D conversion section 305b. Both digital signals are outputted to reference signal correlation section 306, frequency difference detection section 313, and reception AGC control section 314.
Frequency difference detection section 313 detects a phase difference .phi. in carriers between a mobile station and a base station, and adjusts a frequency in frequency synthesizer section 304 to a predetermined value using a control value to cancel the phase difference .phi..
Reception AGC (Automatic Gain Control) control section 314 calculates the time average of each of digital baseband signals I(t) and Q(t) to obtain a received signal level, and controls the reception gain in frequency conversion section 302 using a control value for keeping the received signal level constant. Thus, in frequency conversion section 302, the output level is held constant not depending on the variations of input level, which keeps the received signal level constant.
Reference signal correlation section 306 is to detect the correlation of a received signal and a reference signal. To detect the correlation is to multiple the received signal by the reference signal. The correlation is detected to remove the reference signal by multiplying transmission data by the reference signal again because the transmission data from a base station is multiplied by the reference signal already to transmit.
To multiply as decried above is to obtain an exclusive OR of a received signal and a signal of cos (.omega.ct+.phi.) or a signal of sin (.omega.ct+.phi.) in an EXOR circuits (exclusive-OR circuits ) 303a and 303b illustrated in quadrature demodulation section 303, or to multiple 0 by 1 in data after converting 0 and 1 into +1 and -1 respectively. This manner is the same as in the following explanation.
Further by multiplying a received signal by a reference signal to detect the correlation, the timing of the reference signal included in the received signal can be detected. And the detection of the timing allows a mobile station to acquire the synchronization with a base station with respect to signals.
That is, reference signal correlation section 306 detects the correlation expressed in the formulation (4) below by multiplying a received signal by reference signals PNi(t) and PNq(t) generated in reference signal generation section 307. ##EQU1##
In this correlation processing, Ipn(t) and Qpn(t) signals (correlation signal) indicative of the correlation of each of digital baseband signals I(t) and Q(t) and reference signals PNi(t) and PNq(t) respectively are outputted to channel identification code correlation section 308 and reference signal correlation integration section 311. Correlation signals Ipn(t) and Qpn(t) are expressed in the formulations (5) and (6) below. EQU Ipn(t)={Wd(t).multidot.D(t)+Wp(t)}(cos .phi.-sin .phi.) (5) EQU Qpn(t)={Wd(t).multidot.D(t)+Wp(t)}(cos .phi.+sin .phi.) (6)
Channel identification code correlation section 308 multiplies channel identification code Wd(t) generated in channel identification code generation section 309 by correlation signals Ipn(t) and Qpn(t) (herein obtains exclusive-OR in EXOR circuits 308a and 308b) to detect the correlation of channel identification code Wd(t) and a received signal. The obtained correlation signals IW(t) and QW(t) are expressed in the formulations (7) and (8) below. EQU Iw(t)={D(t)+Wd(t)Wp(t)}(cos .phi.-sin .phi.) (7) EQU Qw(t)={D(t)+Wd(t)Wp(t)}(cos .phi.+sin .phi.) (8)
Channel identification code correlation integration section 310 integrates correlation signals IW(t) and QW(t) in a period of M corresponding to channel identification code Wd(t), and amplitude of a reference signal Wp(t) in a bit duration time and outputs integration signals I.SIGMA.W(t) and Q .SIGMA.W(t) indicative of the integration results respectively to sum of products section 312.
The reason to detect the correlation is explained.
Since an output after the correlation has n times rate that of information data, it is necessary to integrate the outputs to obtain the information data. In detail, when it is assumed that transmission data is "1,0", and a channel identification code is "01010101 . . . " with n times rate that of the transmission data. In this case, the signals change as shown below according to the transmission order.
______________________________________ Transmission signal 10100101 . . . Channel identification code 01010101 . . . Output after correlation 11110000 . . . (correlation signal) Information data (reception data) 1 0 . . . ______________________________________
As shown above, the outputs after the correlation is 11110000 . . . , which has n times rate that of information data. As a method to obtain the information data, there are two methods of choosing one in four and calculation of the average (integration). However in the case of choosing one in four, there may be the possibility to neglect the good data, which may deteriorate the reception quality. Therefore the integration is generally used to obtain the information data.
Integration signals I.SIGMA.W(t) and Q.SIGMA.W(t) outputted from channel identification code correlation section 310 are expressed in the formulations (9) and (10) below, EQU I.SIGMA.w(t)=MD(T).multidot.(cos .phi.-sin .phi.) (9) EQU Q.SIGMA.w(t)=MD(T).multidot.(cos .phi.+sin .phi.) (10)
where T is a bit duration time.
Reference signal correlation integration section 311 integrates correlation signals Ipn(t) and Qpn(t) over M, and outputs integration signals I.SIGMA.pn(t) and Q.SIGMA.pn(t) indicative of the integration results to sum of products section 312. The integration is performed to obtain signals with a bit rate. Therefore an output signal from reference signal correlation integration section 311 and an output signal from channel identification code correlation integration section 310 have the same bit rate to demodulate the information data.
Integration signals I.SIGMA.pn(t) and Q.SIGMA.pn(t) outputted from reference signal correlation integration section 311 are expressed in the formulations (11) and (12) below. EQU I.SIGMA.pn(t)=M.multidot.Wpn(t).multidot.(cos .phi.-sin .phi.)(11) EQU Q.SIGMA.pn(t)=M.multidot.Wpn(t).multidot.(cos .phi.+sin .phi.)(12)
Finally, the sum of products section 312 calculates the sum of products of integration signals I.SIGMA.W(t), Q.SIGMA.W(t), I.SIGMA.pn(t) and Q.SIGMA.pn(t) to generate the demodulated data (information data) expressed in the formulation (13) below. EQU (Demodulated Data)=2M.sup.2 Wpn(t)D(t) (13)
As described above, a base station spreads the transmission data with a reference signal to transmit, and a conventional mobile station despreads the received signal with the reference signal and integrates the obtained signals over a duration period. The mobile station multiplies the despread received signal by a channel identification signal and integrates the obtained signals over a duration period. Then the sum of products of the obtained integration values are calculated to demodulate the information data.
Generally a phase difference .phi. in carries between a base station and a mobile station varies. With respect to the phase difference variation, the primary factor is supposed to be the frequency variation in frequency synthesizer section 304 depending on temperature, and the secondary factor is supposed to be the broadness with a Doppler frequency fD in the power spectrum of a received signal as illustrated in FIG. 4.
The broadness of the spectrum is expressed according to 2fD=2V/.lambda., and depends on a moving speed of a mobile station v and a wavelength of a transmission wave .lambda.. The faster the moving speed becomes, or the shorter wavelength becomes (the higher Doppler frequency fD becomes), the larger the broadness becomes.
The effect of primary on the phase variation in a short time is neglected because it is caused by temperature changes. Regarding the secondary factor, high Doppler frequency fD affects the phase variation in a short time, while low Doppler frequency fD does not affect it much.
According to the facts described above, in the case where the phase variation caused by the primary factor and the secondary factor is small and integration signals I.SIGMA.pn(t) and Q.SIGMA.pn(t) outputted from reference signal correlation integration section 311 do not vary much, in other words, reference signal correlation integration outputs do not vary much, it is not necessary to perform the integration processing in reference signal correlation integration section 311.
However, in a conventional configuration illustrated in FIG. 1, the reference signal correlation integration processing is performed during the reference signal correlation integration outputs do not vary much, which results in increases of consumed power.